The present invention relates to the reproduction of an image or a document having variable grey density, to obtain a result having a plurality of different grey levels starting with an image background level or reproduction medium level giving, for example, a white level (or alternatively a black level), and using two different degrees of medium colouring corresponding to black level and white level respectively, and varying the ratio between the amount of black and white in a given area allocated to each elementary image point (pixel), as a function of the desired grey level for that pixel.
An article entitled "Electronic Halftones" by R. L. Hallows, Jr., and R. J. Klensh of RCA appearing in IEEE Spectrum, October 1968, describes a system for reproducing an image in halftones wherein an image originally of varying grey density is reproduced in the form of black and white dots of different areas. In the system described, the original image is available on the screen of a first cathode ray tube (CRT), while the halftone processed image is made available on the screen of a second CRT whence it could be photographed to obtain a printer's plate. The successive dots of variable area making up the halftone image on the screen of the second CRT are composed by adding two sinusoidal voltages in quadrature to the usual horizontal and vertical deflection voltages. The amplitude of the oscillations increases with time causing the electron beam spot to describe a spiral of increasing radius about the centre of the pixel. The growth of the spiral is interrupted after a length of time corresponding to the brilliance of the corresponding pixel on the first CRT screen.
Halftone reproduction is thus obtained according to the article by moving the electron beam spot over an increasing proportion of the area allocated to each pixel thereby varying the size of the dot marked in said area.
The article "Electronic Color Separation with Laser Light Sources" by Dietrich Meyerhofer, A. Williams Stephens and John J. Walsh, appearing in IEEE Transactions on Communication Technology, vol COM-18, No. 4, August 1970, indicates that halftone images may be obtained from an intensity modulated laser beam. In the experimental device described, a laser having a Gaussian energy distribution is used together with a high gamma film medium to obtain dots whose sizes are related only to the intensity of the laser beam.